Variations of the orbital eccentricity distributions as a function of the stellar age in M 31 and MW HESTIA galaxies. Distributions were normalised by the maximum value at a given age. The top row corresponds to all in situ stars while the bottom corresponds to in situ stars > 1 kpc away from the galactic plane mostly excluding the disc component. White vertical lines highlight five of the most significant mergers (M1–M5), in terms of the stellar mass ratio. The mean eccentricity values are shown by the yellow (top) and black (bottom) lines. Similar to the rotational velocity behaviour (see Fig. 14), the distributions show several episodes of a sharp increase in the eccentricity which coincide with the mergers.